Aircraft spark plug



Dec. 14, 1943.

H. RABEZZANA 2,336,569

AIRCRAFT SPARK PLUG Filed Oct. 13, 1941 3g I I "g 'l l i f Cttornegs Patented Dec. 14, 1943 UNITED STATES PATENT OFFICE AIRCRAFT SPARK PLUG Hector Rabenana, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a. corporation ot Delaware Application October 13, 1941, Serial No. 414,746

1 Claim. (Cl. 123-169) My invention relates to spark plugs designed to ignite the mixture of air and hydrocarbon supplied to and burned within the cylinder of an internal combustion engine, and particularly to socalled shielded spark plugs and to the current supply leads associated therewith; the purpose of the invention being to prevent the emanation of high frequency electro-magnetic waves and other disturbing inuences, due to high tension oscillating current supplied through the leads and whereby the combustible mixture is ignited. Such oscillations if not inhibited as by Suppressors of high resistance included in the supply conductors leading to the plugs, or by enclosing the leads and plugs in metallic sheaths which are grounded to the engine structure with which the plugs are used, interfere with and may prevent the proper operation radio receiving and other sensitive apparatus carried by an aircraft propelled by the engine with which the plugs are used; because of the noise due to the high tension high frequency alternating or varying current in the supply conductors leading to the spark plugs or rather, and more definitely speaking, to the central insulated electrodes of the spark plugs of the engine which propels the aircraft. The invention tu which this present application relates includes various features designed to prevent the high tension current made use of as aforesaid from interfering with the proper operation of adjacent radio and like sensitive devices; and it includes the use of both a high resistance oscillation suppressor and shielding means such as is hereinbefore referred to, associated with each spark plug and with the supply conductor leading to the central insulated electrode thereof.

The drawing accompanying and forming a part of this specification illustrates the preferred form of my invention, and therein:

Figure l is a view showing a section of my improvided spark plug upon a longitudinally extending central plane, and about three times its natural and usual size.

Figure 2 is a view showing a corrugated annular washer and a cap for holding the same in place, located at one end of a short tubular sleeve of ceramic material; there being a like cap and washer at each end of the sleeve.

Figure 3 is a view, further enlarged, showing a piece of the central insulated electrode of my improved spark plug in side elevation.

Figure 4 is a view showing a section upon a transverse plane indicated by the line 4 4, Figure 3.

Referring now to the drawing, the numeral I designates a hollow metallic outer shell or casing circular in cross-section and made commonly of steel, the same being externally threaded at its lower end as shown so as to screw into a correspondingly threaded spark plug opening in the cylinder of an internal combustion engine; and the numeral II designates a tubular shield made of steel or equivalent material and threaded at its lower end, as at I2, so as to screw into the upper end of the casing I il as shown. This shield tube surrounds and extends above the insulating member I3 of the plug, which is supported within the casing I0, and is externally threaded as at H to facilitate the attachment t0 the shield and thus to the plug casing IU of a metallic conduit, not shown, through which the conductor or lead whereby high tension current is supplied to the insulated central electrode I5 of the plug extends. The shield II has a tubular lining or sleeve I6 made of glass, porcelain or equivalent ceramic material; and cushioning devices are provided at each end of this sleeve to protect it from injury. These cushioning devices, they being of the same construction at each end of the sleeve, comprise a thin metallic ring I 1 having integral depending legs I8 which grasp the ends of the sleeve and space it slightly from the interior of the shield II, see Figure l, and corrugated washers i9 of thin metal at the extremities of the sleeve which may yield slightly as the shield is screwed into the shell I0; to grasp the sleeve between an in- Wardly extending ledge 2D at the upper end of the shield Ii and a metallic washer 2l against which the lower end of the threaded portion I2 of the shield abuts when the parts are assembled. This washer, as will be seen, supports the lower one of the cushioning devices, and it engages the upper end of a resilient C-shaped spring washer 22 whereby the insulating member I3 is held in place within the casing IIl as is usual in spark plug constructions.

Extending longitudinally of the insulating member I3 is a passage having a lower portion 23 through which the insulated electrode 24 of the plug extends, and a larger upper or outer portion 25 within which a high resistance oscillation suppressor 26 is housed; the two passages being separated by an internal ledge 2l which provides support for the enlarged upper end of the insulated electrode. A sealing member 28 of plastic conductive material is introduced into the passage 25 and compacted about the upper end of the electrode 24 during the manufacture of the plug to hold the electrode in place, and to prevent leakage along the electrode when the plug is in understood.

2 assauts use. A cup-shaped metallic abutment 20 is pressed into engagement with the upper end of the sealing member as the latter is pressed into engagement with the upper end of the electrode. see Figure l. The suppressor may comprise a granular conductive material mixed with a suitable binder and molded into cylindrical form, as shown, and it is preferably provided with metallic caps Il at its two ends. Its resistance is commonly around from 50 to 100 ohms.

The suppressor 28 is shown as held within the passage 25 in the insulating member by a restraining plug or cap member Il, having a depending tubular part 32 and struck out spring lugs 33 which spring outward and into a groove u within and adjacent the upper end of the insulator i3 as the tubular part 32 of the cap member is pressed into the passage 28. thus providing an interlocking connection between the cap and the upper end of the insulating member; and a spring 35 is interposed between the cap member and the upper end of the suppressor. The conductor or lead through which current is supplied to the plug contacts with the cap 3l, so that current ows through the spring 35 and the suppressor 26 to the insulated electrode 24 as will be The electrode 24 comprises a. central copper core which provides a heat conducting path of' low ristancefrom the spark gap at the lower end of the plug upward therefrom, and a sheath 38 of nickel alloy enclosing said core and protecting the electrode from injury by the high temperature gases within the combustion chamber; and the sheath 36 is shown as provided with a circumferentially arranged series of projections 31 formed, for example, by knurling the sheath; and the purpose of which is -to space the electrode centrally within the passage 23 and prevent the same from coming into violent contact with the interior of said passage; which might otherwise occur with consequent breaking of the lower end of the insulator ii' the plug is subjected to inordinately severe shocks when in use, or during the installation of the plug in the engine with which it is used.

The capacity associated with the ignition device or system herein contemplated is dependent upon the structural features thereof and, in the arrangement herein, depends for the most part and in fact substantially entirely upon a condenser provided by the central insulated electrode o f the plug, and the tubular grounded metallic shield and the shell thereof. The energy stored by this condenser is dependent upon the voltage of the supply current and thel distance these elements are spaced apart from one another, and the stored energy is discharged across the spark gap of the plug.

Assuming a maximum value of 5000 volts in the conductor leading to the spark plug, and a resist-` :1 -f ance of ve ohms at the spark gap after the flow betweentheelectrodeshasbeeninitiatedthe current iiow in the ignition circuit wouii be i000 amperes. This. of course, is very much greater than is required to eiiect ignition of the ccmbustible mixture and. while the excess ci current might perhaps be condoned as it is in the nature of an instantaneous surge of extremely short duration. and the energy loss is small. auch a large current flowing but for an instant would weai` the electrodes away rapidly. When. however, a suppressor having a resistance of around 95 ohms is included in the ignition lead the current now. assuming the same 5000 voltage, will be 50 amperes; which will be abundant to accomplish ignition oi' the combustible mixture, and will at the same time very much lessen the wear or erosion of the electrodes.

Suppressors similar tothe element 20 have heretofore been used in the high tension supply conductors leading to the insulated electrodes ot spark plugs but they have commonly been of high resistance, usually around 10,000 ohms. with the result that the current supplied to the electrode has been very much reduced (it would be halt an ampere in the not unusual case assumed); whereas when a suppressor having a resistance on the order of from 50 to 100 ohms is used the current will insure -proper ignition of the combustible mixture, but will not be great enough to cause excessive wear and erosion of the electrodes.

Having thus described and explained my invention I claim and desire to'secure by Letters Patent: Y 35 In a spark plug of the class described, a hollow casing threaded at its lower end; an insulating member supported within said casing and having a passage extending longitudinally therethrough;

' an insulated electrode within the inner end of said passage and terminating adjacent a second electrode at the inner end of said casing; an oscillation suppressor within the upper end of said passage and through which current ows to said insulated electrode;l acap member inter- .locked with vthe upper end of said insulating member for retaining said suppressor in place within the passage aforesaid; a spring interposed between the upper end of said oscillation suppressor and said cap member and acting to force fsaid suppressor downward, to thereby maintain a yieldable contact between the lower end of said oscillation suppressor and said insulated electrode; a tubular metallic shield extending upward from said hollqw casing beyond the upper end of said insulating member. and which shield surrounds the upper end of said insulating member, said cap member, and said spring; and a tubular sleeve of ceramic material within said metallic shield and enclosing the upper end of said insulating member.

HECTOR RABEZZANA. 

